Currently available FT-IR microscopes capable of generating an infrared image fall broadly into two categories: focal plane array systems, where a relatively large array detector is used to generate an image of a portion of a stationary sample; and single detector systems, where an image of arbitrary size is built up from individual pixels by translating the sample in small steps on a motorized stage in some form of raster scan. The large array based systems are generally obliged, by the low frame rates available from typical detectors, to use a very slow spectrometer scan, often a stepped scan, where an interferometric image of the sample is acquired at each spectrometer scan position before moving on to the next scan position; meanwhile, the sample does not move. The single detectors in contrast have much higher bandwidth, permitting a much more rapid spectrometer scan, typically not stepped, with interferogram data collected continuously during the scan. The present invention is concerned with this latter type of system where the sample is moved stepwise in its own plane after each spectrum acquisition, and concerns the speed and efficiency of data collection. The detector does not have to be a single detector but could be a small array of the type described in EP-A-1 184703 the disclosure of which is incorporated by reference.
One known FT-IR microscope currently generates images (or maps), one pixel at a time by a step and repeat sequence of operations. The cyclic sequence consists of:                step the motorized sample stage to the next sample location;        start the spectrometer scan;        wait for the spectrometer scan to complete and stop;        transfer the data;        step the stage to the next sample location . . . .        
The sequential nature of the operations produces substantial delays as each operation waits for the previous to complete, with the result that it takes a very long time to collect a reasonably-sized image.